Switch from Python 2 to 3 in intersphinx_mapping setting

docs/conf.py

@@ -320,7 +320,7 @@
 
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
-    'python': ('https://docs.python.org/2', None),
+    'python': ('https://docs.python.org/3', None),
     'django': ('https://docs.djangoproject.com/en/dev/',
                'https://docs.djangoproject.com/en/dev/_objects/'),
 }